1. Field of the Invention
The present invention relates to a vehicle ventilation duct for letting out air inside a vehicle passenger room to the outside of a vehicle or vice versa.
2. Description of the Related Art
Vehicles are equipped with a ventilation duct for letting out air inside the passenger room to the outside or vice versa, one of ventilating apparatuses. The ventilation duct comprises a ventilation opening which is opened to the outside of a vehicle passenger room and is covered with a valve element opening outward to the vehicle passenger room and closing inward thereto. The ventilation duct communicates the inside of the vehicle passenger room with the outside of the vehicle. When opening or closing a vehicle door, air flows into the vehicle passenger room, and air with the same volume as that of the flowing-in air pushes the valve element to open. The air having pushed the valve element to open is let out to the outside of the vehicle passenger room by way of the ventilation opening.
When the valve element is opened, noises such as road noises, pattern noises and brake noises have come into the vehicle passenger room from the outside of the vehicle by way of the ventilation opening. Japanese Unexamined Patent Publication (KOKAI) No. 15-182,344, Japanese Unexamined Patent Publication (KOKAI) No. 2001-130,447 and Japanese Unexamined Patent Publication (KOKAI) No. 2002-172,927 disclose inventions which were developed in order to reduce the noises.
Japanese Unexamined Patent Publication (KOKAI) No. 15-182,344 discloses a ventilation duct equipped with a resonator. FIG. 11 illustrates a cross-sectional view of the ventilation duct set forth in the publication. As shown in FIG. 11, a ventilation duct 100 comprises a duct body 101, a sheet-shaped valve element 102, and a resonator 103. As depicted with the blank arrow of the drawing, noises come into a resonator chamber 105 of the resonator 103 by way of a minor hole 104. The resonator chamber 105 suppresses an acoustic pressure of around 63 Hz, some of the noises, by the Helmholtz's resonance theory.
The ventilation duct disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 15-182,344, however, suppresses the acoustic pressure in the limited frequency band falling in a relatively narrow range only. Accordingly, in order to suppress acoustic pressures over a relatively wide range, it is required to independently dispose a plurality of resonators whose intended suppressing frequency band differs with each other. Consequently, the ventilation duct 100 has enlarged inevitably.
Japanese Unexamined Patent Publication (KOKAI) No. 2001-130,447 discloses an air vent structure with a shielding plate installed in the air outlet passage. The shielding plate has a slant surface for reducing air resistance. The shielding plate shields noises which pass through the air outlet passage so that the air vent structure reduces the noises.
In the air vent structure set forth in Japanese Unexamined Patent Publication (KOKAI) No. 2001-130,447, however, noises which have passed the shielding plate disposed in the air outlet passage are transmitted into a vehicle passenger room as they are. Accordingly, the noises have been transmitted to passengers as annoying noises. Moreover, the air outlet passage is communicated with a ventilation opening which is opened to a vehicle passenger room. Consequently, noises have reached passengers directly from the ventilation opening so that the passengers feel them as louder noises which have not been attenuated at all.
Japanese Unexamined Patent Publication (KOKAI) No. 2002-172,927 discloses a ventilation duct in which an air passage opening is formed in the side wall of the air passage and a sound absorption member is disposed on an inner peripheral surface of the air passage. The sound absorption member absorbs noises which transmit in the air passage so that the noises are reduced. Moreover, depending on the positions of the air passage opening formed in the side wall of the air passage, noises do not run into passengers who are sitting in a vehicle passenger room directly. As a result, the level of noises felt by the passengers annoying has fallen.
The ventilation duct set forth in Japanese Unexamined Patent Publication (KOKAI) No. 2002-172,927 suffers from a problem that no sufficient air outlet can be obtained because the size of the air passage opening is limited. That is, the opening magnitude of the air passage opening can be enlarged unless the protrusion of the sound absorbing chamber is expanded. In addition, the flow of air which has come into the sound absorbing chamber through the air passage opening is deflected in the sound absorbing chamber. Accordingly, there arises great air resistance when the air passes through the inside of the sound absorbing chamber. In order to reduce the air resistance, it is required to prolong the distance between the bottom surface of the sound absorbing chamber and the duct body (or to expand the protrusion of the sound absorbing chamber). Note that, however, ventilation ducts are disposed between the exterior of vehicles and the interior. Consequently, the space for disposing ventilation ducts is confined in limited areas. As a result, it is difficult to make the distance longer in certain vehicles.
Thus, it has been difficult for the conventional ventilation ducts to reduce the transmission noises which come into vehicle passenger rooms.